Add psql PROMPT variable for search_path.

The new %S substitution shows the current value of search_path.
Note that this only works when connected to Postgres v18 or newer,
since search_path was first marked as GUC_REPORT in commit
28a1121fd9.  On older versions that don't report search_path, %S is
replaced with a question mark.

Suggested-by: Lauri Siltanen <lauri.siltanen@gmail.com>
Author: Florents Tselai <florents.tselai@gmail.com>
Reviewed-by: Jelte Fennema-Nio <postgres@jeltef.nl>
Reviewed-by: Jim Jones <jim.jones@uni-muenster.de>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/CANsM767JhTKCRagTaq5Lz52fVwLPVkhSpyD1C%2BOrridGv0SO0A%40mail.gmail.com

doc/src/sgml/ref/psql-ref.sgml

@@ -4974,6 +4974,17 @@ testdb=&gt; <userinput>INSERT INTO my_table VALUES (:'content');</userinput>
         </listitem>
       </varlistentry>
 
+      <varlistentry id="app-psql-prompting-S">
+       <term><literal>%S</literal></term>
+       <listitem>
+        <para>
+         The current value of <xref linkend="guc-search-path"/>, or
+         <literal>?</literal> if connected to a server running
+         <productname>PostgreSQL</productname> 17 or older.
+        </para>
+       </listitem>
+      </varlistentry>
+
       <varlistentry id="app-psql-prompting-s">
         <term><literal>%s</literal></term>
         <listitem><para>The name of the service.</para></listitem>